1. Field of the Invention
The present invention relates to an information recording medium for optically recording, erasing, rewriting, and reproducing information, a method for producing the same, and a method for recording/reproducing information with respect to the same.
2. Description of the Related Art
In a phase-change information recording medium, information is recorded, erased and rewritten using a recording layer that is transformed in phase reversibly between a crystal phase and an amorphous phase. When this recording layer is irradiated with a high power laser beam and then is cooled rapidly, a portion thus irradiated is changed to be in an amorphous phase. Similarly, when an amorphous portion of the recording layer is irradiated with a low power laser beam and then is cooled slowly, the portion thus irradiated is changed to be in a crystal phase. Therefore, in the phase-change information recording medium, the recording layer is irradiated with laser beams having powers modulated between a high power level and a low power level, whereby the information layer can be changed freely to be in an amorphous phase or a crystal phase. In the phase-change information recording medium, information is recorded using the difference in reflectivity between an amorphous phase and a crystal phase.
In recent years, in order to enhance the recording density of an information recording medium, various techniques have been studied. For example, there are techniques of recording a smaller recording mark using a violet laser beam and of recording a smaller recording mark by making a substrate thinner on a light incident side while using a lens with a large numerical aperture. A technique of recording/reproducing information with respect to two recording layers using a laser beam incident from one side also has been studied (see JP 12(2000)-36130 A).
In order to decrease the size of a recording mark, it is necessary to shorten an irradiation time of a laser beam used for changing a phase of a recording layer. This requires that the crystallization speed of the recording layer should be high. Furthermore, in order to record/reproduce information with respect to two recording layers, it is required to use a thin recording layer on a light incident side so that sufficient light reaches a recording layer on the back side. However, when the recording layer is thinned, the number of atoms contained in the recording layer is decreased, and the movement of atoms involved in a phase change also is suppressed, which lowers the crystallization speed. Therefore, there is a demand for a material capable of forming a recording layer on which information can be recorded with reliability irrespective of its thinness.
Conventionally, as a material for a recording layer, Gexe2x80x94Sbxe2x80x94Te system materials have been used. According to an experiment by the inventors of the present invention, it is found that, among them, a pseudo binary composition GeTexe2x80x94Sb2Te3 has the highest crystallization speed, and Ge2Sb2Te5 ((GeTe): (Sb2Te3)=2:1) has excellent characteristics. Furthermore, Uno et al. report a recording/reproducing experiment using a Gexe2x80x94Sbxe2x80x94Te recording layer with a thickness of 6 nm (M. Uno, K. Nagata and N. Yamada, xe2x80x9cThinning Limitation of Gexe2x80x94Sbxe2x80x94Te Recording Film for High Transmittance Mediaxe2x80x9d, Proc. of PCO""99. 83-88). In this experiment, information was erased at a linear velocity of 9 m/s using a laser with a wavelength of 660 nm, and a satisfactory erasure ratio (30 dB) was obtained.
However, when the inventors conducted an experiment on a Gexe2x80x94Sbxe2x80x94Te system material, using a violet laser with a wavelength of 405 nm, this material was found to be insufficient for use on a light incident side. Therefore, in a conventional recording layer, it was difficult to realize an information recording medium having a two-layered structure, with respect to which information is recorded/reproduced using a violet laser.
JP 2(1990)-147289 A reports that an information recording medium is obtained that has excellent repeated-recording/erasing characteristics and less change in an erasure ratio with time by adding Sb to Texe2x80x94Gexe2x80x94Sn of a recording layer so as to limit the content of each element. However, this is an experimental result in the case where an information recording medium includes only one recording layer, and the recording layer is thick (i.e., 30 to 100 nm). This publication does not show the effects of addition of Sn in the case where the recording layer is thinned.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a high-density recordable information recording medium having two recording layers, a method for producing the same, and a method for recording/reproducing information with respect to the same.
In order to achieve the above-mentioned object, an information recording medium of the present invention includes: a first substrate; a second substrate disposed so as to be opposed to the first substrate; a first information layer disposed between the first substrate and the second substrate; a second information layer disposed between the first information layer and the second substrate; and an intermediate layer disposed between the first information layer and the second information layer, wherein the first information layer includes a first recording layer that is transformed in phase reversibly between a crystal phase and an amorphous phase with a laser beam radiated from the first substrate side, the second information layer includes a second recording layer that is transformed in phase reversibly between a crystal phase and an amorphous phase with the laser beam, and the first recording layer contains Ge, Sn, Sb, and Te, and has a thickness of 9 nm or less.
In the above-mentioned information recording medium, the first recording layer may be made of a material represented by a composition formula: (Gexe2x80x94Sn)ASbBTe3+A, where 2xe2x89xa6Axe2x89xa622 and 2xe2x89xa6Bxe2x89xa64. This composition formula represents that Ge and Sn are contained in the material by 100*A/(2A+B+3) atomic % in total. According to this constitution, even when the first recording layer is made thin, satisfactory recording/erasing characteristics are obtained with a violet laser. By setting 2xe2x89xa6A, an amplitude of a signal can be increased. Furthermore, by setting A xe2x89xa622, a decrease in crystallization speed can be prevented. By setting 2xe2x89xa6B, Te, which has a low melting point, can be prevented from being precipitated when a phase change between a crystal phase and an amorphous phase is effected. In the case of 2 less than B, an excess amount of Sb is added to the material represented by (Gexe2x80x94Sn)ASb2Te3+A. This excess amount of Sb functions to increase a crystallization temperature to enhance thermal stability of a recording mark, and suppressing the movement of a substance during repeated-recording.
In the above-mentioned information recording medium, a content of Sn in the first recording layer may be 25 atomic % or less. The content of Sn preferably is 0.1 atomic % or more. According to this constitution, even when the first recording layer is made thin, a satisfactory erasure ratio is obtained with a violet laser. Furthermore, by adjusting the content of Sn in the first recording layer and B, the crystallization speed and the crystallization temperature of the first recording layer can be controlled.
In the above-mentioned information recording medium, a transmittance Tc (%) of the first information layer in a case where the first recording layer is in a crystal phase, and a transmittance Ta (%) of the first information layer in a case where the first recording layer is in an amorphous phase may satisfy 40xe2x89xa6(Tc+Ta)/2 with respect to a laser beam having a wavelength in a range of 390 nm to 430 nm. According to this constitution, satisfactory recording/erasing characteristics also are obtained in the second information layer.
In the above-mentioned information recording medium, the transmittance Tc (%) and the transmittance Ta (%) may satisfy 0xe2x89xa6|Tcxe2x88x92Ta|/Tcxe2x89xa60.15 (more preferably, 0xe2x89xa6|Tcxe2x88x92Ta|/Tcxe2x89xa60.05) with respect to a laser beam having a wavelength in a range of 390 nm to 430 nm. According to this constitution, a change in recording sensitivity of the second information layer can be decreased irrespective of a recorded state of the first information layer
In the above-mentioned information recording medium, the first information layer further may include first and second dielectric layers and a first reflective layer, and the first reflective layer, the second dielectric layer, the first recording layer and the first dielectric layer may be disposed in this order from the intermediate layer side to the first substrate side. According to this constitution, by varying a material and a thickness of the dielectric layers and the reflective layers, the light absorptivity of the first recording layer, and the transmittance and the reflectivity of the first information layer can be controlled.
In the above-mentioned information recording medium, the first information layer further may include a third dielectric layer disposed between the first reflective layer and the intermediate layer. According to this constitution, by varying a material and a thickness of the third dielectric layer, the transmittance of the first information layer can be increased.
In the above-mentioned information recording medium, a refractive index of the third dielectric layer may be 2.3 or more with respect to light having a wavelength in a range of 390 nm to 430 nm.
In the above-mentioned information recording medium, grooves for tracking control may be formed on the intermediate layer.
In the above-mentioned information recording medium, the first information layer further may include an interface layer disposed at at least one interface selected from the group consisting of an interface between the first dielectric layer and the first recording layer, an interface between the first recording layer and the second dielectric layer, an interface between the second dielectric layer and the first reflective layer, and an interface between the first reflective layer and the third dielectric layer. According to this constitution, the movement of a substance between layers can be suppressed, so that an information recording medium with high reliability is obtained.
In the above-mentioned information recording medium, a thickness of the first reflective layer may be in a range of 5 nm to 15 nm. According to this constitution, the transmittance Tc (%) and Ta (%) of the first information layer can be enhanced, and the first recording layer easily can be changed to be in an amorphous phase by rapidly diffusing heat generated therein. When the first reflective layer is too thin, its heat diffusion function is insufficient, and when it is too thick, the transmittance of the first information layer becomes insufficient. Therefore, the thickness of the first reflective layer preferably is set in a range of 5 nm to 15 nm.
In the above-mentioned information recording medium, a thickness of the first substrate may be in a range of 10 xcexcm to 700 xcexcm. According to this constitution, by varying a numerical aperture NA of an objective lens, the length of a recording mark and the interval between recording marks can be optimized in accordance with the shape of grooves of the first substrate and recording/erasing/reproducing conditions.
In the above-mentioned information recording medium, grooves for tracking control may be formed on the first substrate.
In the above-mentioned information recording medium, a thickness of the second substrate may be in a range of 500 xcexcm to 1300 xcexcm. According to this constitution, by varying a numerical aperture NA of an objective lens, the length of a recording mark and the interval between recording marks can be optimized in accordance with the shape of grooves of the first substrate and recording/erasing/reproducing conditions. The thickness of the second substrate is selected so that the thickness of the information recording medium becomes about 1200 xcexcm. In the case where the thickness of the first substrate is about 100 xcexcm, the thickness of the second substrate is set to be about 1100 xcexcm. Furthermore, in the case where the thickness of the first substrate is about 600 xcexcm, the thickness of the second substrate is set to be about 600 xcexcm.
In the above-mentioned information recording medium, grooves for tracking control may be formed on the second substrate.
In the above-mentioned information recording medium, the second information layer further may include fourth and fifth dielectric layers and a second reflective layer, and the second reflective layer, the fifth dielectric layer, the second recording layer and the fourth dielectric layer may be disposed in this order from the second substrate side to the intermediate layer side. According to this constitution, by varying a material and a thickness of the dielectric layers and the reflective layers, the light absorptivity of the second recording layer and the reflectivity of the second information layer can be controlled.
In the above-mentioned information recording medium, the second information layer further may include an interface layer disposed at at least one interface selected from the group consisting of an interface between the fourth dielectric layer and the second recording layer, an interface between the second recording layer and the fifth dielectric layer, and an interface between the fifth dielectric layer and the second reflective layer.
Furthermore, a method for producing an information recording medium of the present invention is a method for producing an information recording medium including first and second substrates, first and second information layers, and an intermediate layer, the method including the processes of: (a) forming the second information layer on the second substrate; (b) forming the intermediate layer on the second information layer; (c) forming the first information layer on the intermediate layer; and (d) attaching the first substrate on the first information layer, wherein the first information layer includes a first recording layer that is transformed in phase reversibly between a crystal phase and an amorphous phase with a laser beam radiated from the first substrate side, the second information layer includes a second recording layer that is transformed in phase reversibly between a crystal phase and an amorphous phase with the laser beam, and the process (c) includes the process of forming the first recording layer to a thickness of 9 nm or less, using a base material containing Ge, Sn, Sb, and Te. According to this production method, the information recording medium of the present invention can be produced easily. Furthermore, according to this production method, since the first substrate is stacked after the second information layer and the first information layer are formed, an information recording medium provided with a first thin substrate can be produced easily.
According to the above-mentioned production method, in the process (c), the first recording layer may be formed by sputtering using sputtering gas containing argon gas or krypton gas. According to this constitution, an information recording medium with excellent repeated-recording characteristics can be formed easily.
According to the above-mentioned production method, the sputtering gas further may contain at least one gas selected from the group consisting of oxygen and nitrogen.
According to the above-mentioned production method, the first recording layer may be formed at a film-formation speed in a range of 0.1 nm/second to 10 nm/second. According to this constitution, the variations in thickness of the first recording layer can be decreased, and the first recording layer can be produced with good productivity in a short period of time.
According to the above-mentioned production method, in the process (b), grooves for tracking control may be formed on a surface of the intermediate layer.
Furthermore, according to the above-mentioned production method, the first information layer further may include a first reflective layer disposed on the intermediate layer side from the first recording layer, and the process (c) may include the process of forming the first reflective layer in a range of 5 nm to 15 nm.
Furthermore, a recording/reproducing method of the present invention is a method for recording/reproducing an information signal by irradiating an information recording medium with a laser beam, wherein the information recording medium is the above-mentioned information recording medium of the present invention, the laser beam is incident from the first information layer side of the information recording medium, in the second information layer of the information recording medium, information is recorded/reproduced with the laser beam transmitted through the first information layer, and a wavelength of the laser beam is in a range of 390 nm to 430 nm. According to this recording/reproducing method, high-density recording can be conducted with high reliability.
In the above-mentioned recording/reproducing method, a linear velocity of the information recording medium in recording/reproducing information may be in a range of 1 m/second to 50 m/second. According to this constitution, the length of a recording mark and the interval between recording marks can be optimized in accordance with the structure of an information recording medium and recording/reproducing conditions, and a high transfer rate can be realized.
In the above-mentioned recording/reproducing method, the laser beam may be a laser beam condensed by an objective lens with a numerical aperture NA in a range of 0.4 to 1.1. According to this constitution, the length of a recording mark and the interval between recording marks can be optimized in accordance with the thickness of the first substrate or the second substrate, the shape of grooves, and recording/erasing/reproducing conditions, and a high transfer rate can be realized.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.